Various controls for protecting a hydrostatic transmission from over-pressure are known. A hydrostatic transmission has a pair of hydraulic displacement devices connected in a closed loop circuit, with one of the displacement devices being of variable displacement and normally operating as a pump to supply fluid under pressure in a high pressure line side of the closed loop circuit to the other displacement device functioning as a motor. A low pressure line of the closed loop circuit directs fluid from the motor back to the pump. The controls for protecting the hydrostatic transmission from over-pressure have included pressure relief valves, pressure compensators and pressure limiters.
The high pressure relief valve cross-ports fluid from the high pressure line to the low pressure line to relieve the over-pressure condition. The high pressure relief valve reacts quickly, preventing excessive overshoot in the pressure in the high pressure line. However, there is unnecessary power consumption and wasted heat by the pumped flow being recirculated without use in operation of the motor.
The pressure compensator control, as typically used, limits a control pressure applied to servo means associated with the control of pump displacement when the system pressure in the high pressure line reaches a predetermined value. This limiting of control pressure for the servo means results in a reduction of pump stroke and the resulting limiting of system pressure. The pressure compensator results in less power consumption and heat generation than the high pressure relief valve control, but is inherently slow due to the required destroking of the pump. This results in significant system over-pressure under transient conditions where a load is applied quickly to the hydrostatic transmission.
The pressure limiter control utilizes a pilot valve which senses system pressure in the high pressure line and, when that pressure exceeds a predetermined value, the pilot valve opens and ports fluid to the servo means for destroking the pump. The destroking of the pump results in a reduction of the volume of fluid being pumped and a limiting of the system pressure and, thereby, minimizes power consumption and heat generation. The pressure limiter does not have the fast response of the high pressure relief valve, but is an improvement over the older pressure compensator control, due to the fact that high pressure fluid in the high pressure line is used to destroke the pump and the destroking occurs more rapidly.
It is known to use both high pressure relief valves and pressure limiters in the same hydrostatic transmission to overcome the problems inherent in use of only one or the other of such controls. However, such use of high pressure relief valves and pressure limiters has presented another difficulty due to the two controls being separate and the settings of the controls are subject to variations due to manufacturing practices and degradation from use. Normally, the setting of the pressure limiter is at a value lower than the setting of the high pressure relief valve, so that the pressure limiter will first function upon system pressure reaching a predetermined value and, if the pressure goes to a certain value higher, the high pressure relief valve will then operate. With the use of separate controls, the settings of the two control devices must be separated sufficiently to assure that the high pressure relief valve will not operate before the pressure limiter. The invention disclosed herein combines the high pressure relief valve and the pressure limiter valve into one valve assembly, resulting in a high pressure relief valve whose setting cannot go below that of the pressure limiter valve.
A hydrostatic transmission has a charge pump for supplying make-up fluid to the closed loop circuit through charge check valves which open when charge pressure exceeds the pressure in the low pressure line of the hydrostatic transmission. It is also known to utilize a bypass valve with the hydrostatic transmission which cross-connects the two pressure lines of the closed circuit when a device, such as a vehicle utilizing the hydrostatic transmission in the drive for the vehicle, is being towed and the displacement device normally operating as a motor is functioning as a pump. These additional functions have normally been accomplished by valves independently positioned in the circuitry associated with the hydrostatic transmission. A multi-function valve embodying the invention disclosed herein incorporates these valve mechanisms into the same valve body which has the high pressure relief valve and the pressure limiter, resulting in a valve that replaces several independent valves and which are independently mounted to result in a reduction of passageways in an end cap of the variable displacement device and an elimination of external manifolds, hoses and pressure compensator control housings.
Valve structures are available which incorporate components for performing plural control functions in association with a hydrostatic transmission. One of these valve structures provides high pressure relief as well as a charge check valve and adjustable mechanism provides the bypass function. This valve structure does not combine the functions of high pressure relief and pressure limiter control. Another available valve assembly has the charge check valve and the high pressure relief valve incorporated into the same structure, but does not provide structure functioning as a pressure limiter, nor does the structure provide the bypass function.